Combustion chambers of gas-fired hot water heaters are constructed below the bottom wall of the inner tank which contains water to be heated. A flue normally extends centrally in the inner tank from the bottom wall. The combustion chamber if formed by a circumferential skirt or wall disposed below the lower tank and a bottom wall which is elevated from a floor support surface of the hot water heater by a bottom pan. The side walls and bottom wall of the combustion chamber are usually constructed from hot or cold rolled steel which is very thin. This steel is usually treated to remove scales and often this steel is not of uniform thickness. Due to the poor quality of this steel and the thickness thereof, there is excessive heat loss through the side wall and bottom wall of the combustion chamber and this reduces the efficiency of the hot water heater.